The present invention relates to a semiconductor memory and more particularly to a nonvolatile semiconductor memory in which each memory cell includes a memory transistor having a floating gate.
In a semiconductor memory of this type, data-writing is carried out by applying a high programming voltage between a control gate and a drain of a selected memory cell to inject carriers of holes or electrons to the floating gate thereof. A time period for completing carrier injection into the floating gate (called hereinafter "injection period") is determined by a device structure of the memory transistor such as a thickness of a gate insulating film, a transistor size, etc.
The semiconductor memory according to prior art does not have means for detecting completion of the carrier injection. For this reason, a programming time period during which the high programming voltage is applied between the control gate and drain of the memory cell is designed to be considerably long as compared to the injection period in order to ensure the data writing. An unnecessary power is thereby consumed during the programming time period after the undetected completion of injection. Moreover, an address cannot be changed until the programming time period elapses, so that a large time is required in total to write data into all of the memory transistors.